CYP2C9 rs7900194: Understanding the *8 Variant
The rs7900194 single nucleotide polymorphism is a coding variant located within the CYP2C9 gene on chromosome 10, characterized by a c.449G>A change that results in an amino acid substitution (p.Arg150His). In pharmacogenomics, this variant defines the decreased-function CYP2C9*8 allele. Carrying this variant is statistically associated with reduced clearance and lower maintenance dose requirements for medications metabolized by the CYP2C9 enzyme, notably the anticoagulant warfarin.
What each genotype means
Typical enzyme activity
You carry two copies of the standard G allele, which corresponds to the reference functional CYP2C9 enzyme at this locus. Research shows individuals with this genotype typically clear CYP2C9-metabolized medications such as warfarin, phenytoin, and sulfonylureas at expected baseline rates, assuming no other variants are present. Always consult your healthcare provider or pharmacist regarding any medication dosing decisions.
Found in over 95% of individuals globally, including the vast majority of European, Asian, and African populations.
Decreased enzyme activity
You carry one copy of the A variant allele, historically classified in pharmacogenomic guidelines as the CYP2C9*8 allele (c.449G>A; p.Arg150His). Clinical evidence from CPIC demonstrates this allele confers reduced CYP2C9 activity, which is statistically associated with slower clearance and lower dose requirements for drugs like warfarin. You should discuss your specific dosing and potential adjustments with your clinician or pharmacist.
Carried by approximately 4% to 10% of individuals of African ancestry, but is extremely rare in individuals of European or East Asian descent.
Substantially reduced enzyme activity
You carry two copies of the decreased-function A allele (CYP2C9*8 homozygote). Clinical research and pharmacogenetic guidelines indicate this genotype leads to markedly reduced enzyme clearance, substantially lowering the dose required for substrates like warfarin and increasing risk of adverse drug reactions if given standard amounts. It is critical to review medication choices and personalized dosing with your clinician or pharmacist before starting therapy.
Extremely uncommon, occurring in well under 1% of individuals worldwide and observed almost exclusively in populations with African ancestry.
Genetic Identity and Biological Mechanism
The single nucleotide polymorphism rs7900194 is located in exon 3 of the CYP2C9 gene on chromosome 10. At this specific genomic position, the typical nucleotide guanine (G) is substituted by adenine (A), classified as c.449G>A (p.Arg150His). In the standardized nomenclature curated by the Pharmacogene Variation Consortium (PharmVar) and Clinical Pharmacogenetics Implementation Consortium (CPIC), this alteration serves as the core defining variant for the CYP2C9*8 allele. The substitution replaces an arginine residue with a histidine at position 150 of the CYP2C9 peptide chain. While in vitro functional assessments have occasionally shown varying substrate activity, in vivo studies demonstrate that the alteration creates a decreased-function enzyme that processes certain therapeutic substrates more slowly than the reference protein.
The Role of the CYP2C9 Enzyme in Metabolism
The CYP2C9 gene encodes an essential phase I drug-metabolizing enzyme belonging to the cytochrome P450 superfamily, localized primarily within the endoplasmic reticulum of hepatocytes. As detailed by [MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/cyp2c9/), CYP2C9 is responsible for the oxidative clearance and breakdown of a wide variety of endogenous compounds and clinically vital pharmaceuticals. Its substrates include oral anticoagulants like warfarin, nonsteroidal anti-inflammatory drugs (NSAIDs) such as celecoxib and ibuprofen, antiepileptic agents like phenytoin, and oral sulfonylurea antidiabetic medications. When CYP2C9 enzyme function is reduced by genetic variations, drugs that rely on this pathway for clearance can remain active in the systemic circulation for prolonged periods, altering drug exposure profiles.
Clinical Evidence and Warfarin Associations
Clinical pharmacogenomics research indexed by [PharmGKB](https://www.clinpgx.org/summaryAnnotation/981238341) and the Association for Molecular Pathology (AMP) demonstrates that carriers of the CYP2C9*8 allele (rs7900194 A) exhibit significantly lower clearance of S-warfarin. Multiple observational and clinical cohort studies have found that individuals carrying this variant require a lower maintenance dose of warfarin to achieve targeted therapeutic anticoagulation compared to individuals homozygous for the reference allele (CYP2C9*1). Because S-warfarin is the more potent enantiomer cleared by CYP2C9, impaired clearance increases the potential risk of supratherapeutic international normalized ratio (INR) values and associated bleeding if conventional dosing algorithms are applied. Clinical practice guidelines include CYP2C9*8 as a recommended Tier 1 allele for multiethnic clinical testing panels.
Population Distribution and Ancestry Patterns
Unlike classic European-predominant alleles such as CYP2C9*2 and CYP2C9*3, the rs7900194 variant is enriched predominantly in individuals of African ancestry. According to consensus recommendations from the [Association for Molecular Pathology](https://pmc.ncbi.nlm.nih.gov/articles/PMC7057225/), the CYP2C9*8 allele occurs at an estimated frequency between 0.02 and 0.05 (2% to 5%) in African American and sub-Saharan African populations. Conversely, the variant is extremely rare or virtually absent (minor allele frequency well below 0.1%) among populations of European, East Asian, or South Asian descent. Historically, panels that restricted genotyping to *2 and *3 captured the vast majority of variation in European populations but missed approximately 75% of functional variation in individuals of African ancestry, underscoring the critical need to test rs7900194.
Interpreting Results in Clinical Context
Understanding your rs7900194 genotype provides informative insight into how your liver metabolizes specific pharmaceuticals, but it cannot be used on its own to predict health outcomes or diagnose any disorder. Genetic variability in CYP2C9 represents only one component of medication response; other contributing factors include concurrent medications, renal function, liver health, age, diet, and variation in complementary genes like VKORC1 and CYP4F2. A direct-to-consumer or laboratory report indicating an rs7900194 variant should never prompt an individual to adjust, initiate, or discontinue any prescribed therapy independently. Any interpretation of pharmacogenomic testing and resulting dosage optimizations must be evaluated in collaboration with a licensed physician or clinical pharmacist.
How common is this variant?
The rs7900194 variant has a minor allele frequency between 2% and 5% in African and African American populations, while remaining exceedingly rare (<0.1%) in European and Asian cohorts.
Frequently asked questions
What does having the CYP2C9 rs7900194 variant mean for me?
Having the rs7900194 variant (the defining marker of the CYP2C9*8 allele) indicates that your body may metabolize certain medications more slowly than average. This statistical association is particularly relevant for drugs like warfarin, where slower clearance can lead to drug accumulation. However, possessing the variant does not mean you have a disease or medical condition.
Does rs7900194 increase my risk of bleeding on warfarin?
Because the variant slows the metabolism of active warfarin, carriers treated with standard starting doses may reach higher-than-expected levels in the blood, which can elevate bleeding risk. To mitigate this risk, pharmacogenomic dosing algorithms factor in CYP2C9 genotypes to calculate a safer, personalized maintenance dose.
Why is rs7900194 especially important for people of African descent?
Early pharmacogenetic tests primarily screened for CYP2C9*2 and *3, which are common in European lineages but rare in African lineages. The rs7900194 variant is the most common decreased-function CYP2C9 variant found in African ancestries, making its inclusion essential to avoid health disparities in personalized medicine.
Should I change my medication dose based on my genotype?
No, you should never modify your prescription medication doses based solely on a genetic test result. Medication dosage is influenced by clinical factors, lifestyle, diet, and drug interactions; any potential dose adjustments should only be made under the supervision of a prescribing physician or pharmacist.
Sources & further reading
Educational information only, last refreshed 9/14/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Get my report — $29Related variants in CYP2C9
Rare variant in the CYP2C9 gene potentially impacting enzyme activity and drug metabolism; clinical significance requires further validation.
Coding variant conferring the CYP2C9*3 allele (often mislabeled as *4 in older literature) with decreased enzymatic activity affecting clearance of drugs such as warfarin and phenytoin.
Missense mutation defining the CYP2C9*5 allele that dramatically lowers metabolic activity towards oral anticoagulants and anti-epileptic medications.
A rare variant in the CYP2C9 gene that may impact the metabolism of various clinical drugs.
The CYP2C9*2 variant (p.Arg144Cys) results in reduced enzymatic activity, significantly impacting the metabolism and required dosing of substrates such as warfarin and irbesartan.
Defining the CYP2C9*3 (p.Ile359Leu) allele, this variant markedly decreases enzymatic activity, requiring reduced dosages for warfarin and phenytoin to prevent toxicity.